Radioactive screen.
An example of the element Uranium

Radioactive screen.
I'm pretty sure this screen was made by gluing the same ore described in the previous sample onto some cardboard. The manual that came with the chemistry set this was in (see previous sample description) has instructions for how to use the screen to expose photographic film. Source:eBay seller 6tomcatContributor:Theodore GrayAcquired:10 January, 2003Price: $58/chemistry setSize: 1"Purity: <1%